Sliding friction

When two solid objects are in contact and a force is applied to slide one object against the other, sliding friction force resists the motion. If F is the force pushing on an object and Fr is the force of friction, the relationship between F and Fr will determine whether the object will slide or not move at all.

Static friction

If the pushing force F is less than the resistive force of friction Fr (written as F < Fr), there is no motion and the objects remain static with respect to each other. In this case, the friction is considered static friction, which means it is not moving.

Kinetic friction

If force F is greater than friction Fr (written as F > Fr), then the object will slide or move. The friction is considered kinetic friction, which means moving friction.

Pushing force is greater than resistive friction force

Static friction is greater than kinetic

What is interesting is that the static friction that holds an object in place is greater than the kinetic friction that slows down a moving object. In other words, once you start an object moving, the friction decreases from the static friction holding the object in place.

You have seen this in trying to slide a heavy box across the floor. It may be very difficult to move, but once it starts sliding, it is easier to push.

Causes of sliding friction

The causes of sliding friction are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials.

Rolling friction

When a wheel of ball is in contact with a solid surface, and a force is applied to the wheel, static friction will prevent the wheel from sliding. Instead, the wheel will start to roll. Once the wheel is rolling, another type of friction takes over. Rolling friction is the resistive force that slows the wheel's motion on the other solid surface. It is different than static or kinetic friction. Much of rolling friction is caused by adhesion between the surfaces.

Starts rolling

When a force is applied to a wheel is not enough to overcome the static force of friction, the wheel will start to roll.

If the force is greater than the static resistance, the wheel will slide or spin. It will also roll, but not at the same rate as with static friction.

A good example of this is accelerating an automobile on wet pavement. Pushing on the accelerator peddle too hard will cause the wheels to spin, and the car will not move forward as fast as when you push on the gas peddle less.

Slows rolling

Once the wheel is rolling, friction at the point of contact with the other surface slows down the motion of the wheel. Typically, rolling friction is much less than sliding friction. A wheel can roll for some distance before slowing down and stopping.

But there are situations where rolling friction can be large. Trying to ride a bicycle in loose dirt is an example of friction greatly slowing down the rolling motion.

Rolling friction is less than sliding friction

An advantage of rolling friction is that the resistive force is much less than sliding friction.

When the Great Pyramids were being build in ancient Egypt, they used logs as rollers under the giant blocks of granite instead of trying to slide the rocks along the ground.

Rollers reduce friction when moving granite block

Causes of rolling friction

The causes of rolling friction are similar to that of sliding friction. They are molecular attraction or adhesion between the materials, surface roughness of the materials, and deformation resistance in the case of soft materials.

Fluid friction

When a solid object is in contact with a fluid, such as a liquid or gas, and a force is applied to either the object or to the fluid, there is a friction force that resists the motion. Examples where fluid friction occurs are water flowing through a hose, an airplane flying through the atmosphere and oil lubricating moving parts.

Static and kinetic

If the viscosity or thickness of the fluid is great, there may be no movement due to static friction. One example is trying to move heavy grease through a hose. You need to apply a great pressure to finally break the static friction and start the grease moving.

Once a fluid moves through a hose or an object is moving along a fluid, the resistance is considered kinetic friction. The grease will still move much slower than a fluid with low viscosity, like water.

Note that it is also possible to have fluid friction with moving one fluid in contact with another fluid. That subject is usually classified as part of Fluid Dynamics and is not within the scope of our lessons.

Causes of fluid friction

Causes of fluid friction are turbulence effects from surface roughness and deformities, molecular attraction or adhesion between the materials, and deformation resistance of the fluid. This deformation resistance is called its viscosity.

Summary

Friction is a force that resist the motion of the object that is in contact with another object or material. The different types friction are sliding, rolling and fluid friction. When the objects don't move, the friction is called static. When they do move, the friction is called kinetic. The cause of friction is a combination of molecular adhesion, surface roughness, and deformation effects.